Preparation and tribological properties of multi-layer graphene/silicon dioxide composites-based solid lubricant coatings at elevated temperatures

The solid lubricating coatings have an important role in hot metal
forming. However, traditional lubricants can’t be applied to the harsh
working conditions. In this investigation, the novel solid lubricant
coatings including multi-layer graphene (MLG)/silicon dioxide (SiO2)
composites and sodium metaphosphate phosphate were prepared. The
high-temperature tribological properties of the solid lubricant coatings
were investigated by friction and wear tester. The experimental results
showed that SiO2 nanoparticles were evenly grafted by sol–gel method on
the surface of MLG, forming MLG/SiO2 composites. MLG/SiO2 composites were
presented excellent thermal stability at 800°C. In the range of 400–800°C,
the average coefficient of frictions (COFs) were decreased from 0.3936 to
0.3663, and then increased from 0.3663 to 0.4226. Based on the analysis of
wear scar, the lubrication mechanisms of the solid lubricating coatings
were proposed. The low interlayer shear of MLG and the ball bearing of
SiO2 nanoparticles are the main reason for the reduction of COFs. In
addition, the tribo-chemical reaction film formed on the frictional
interface could protect the contact surfaces from severe damages. The
findings would be beneficial for developing novel lubricants for hot metal
forming process.